1. Field of the Invention
The present invention relates to a light-receiving device for carrying out photoelectric conversion; and, in particular, to a back-illuminated type light-receiving device.
2. Related Background Art
A typical back-illuminated type photodiode comprises an n-type semiconductor layer disposed on the front side of a semiconductor substrate, and a p-type semiconductor layer disposed on the n-type layer. A pn junction is formed between the n-type layer and the p-type layer. On the back side of the substrate, an optical lens is provided as a light-receiving portion. Light enters the photodiode by way of the optical lens.
When a reverse bias voltage is applied between the p-type layer and n-type layer in such a photodiode, an i-type layer, which is a depletion region, is provided in the pn junction between the p-type and n-type layers. When light enters the i-type layer after passing through the substrate from the light-receiving portion, electron-hole pairs (carriers) are generated. According to the internal electric field in the i-type layer, the electrons and holes move to the n-type layer and p-type layer, respectively.
When light enters into such a back-illuminated photodiode, the magnitude of light within the i-type layer is larger as the position is closer to the n-type layer. The light exponentially decreases as the position is farther from the n-type layer. Therefore, a larger number of carriers (electron-hole pairs) are generated in the i-type layer as the position is closer to the n-type layer. Thus, the behavior of electron-hole pairs generated near the n-type layer greatly influences output characteristics of the photodiode.
When a electron-hole pair is generated near the n-type layer in a conventional photodiode, the hole travel a longer distance than the electron does. This is because that the distance from the region where the electron-hole pair is generated to the p-type layer is longer than the distance therefrom to the n-type layer. The drift velocity of a hole is smaller than that of an electron. Therefore, a large difference exists between respective times at which the concurrently generated electron and hole reach the n-type layer and p-type layer. The time difference reduces frequency performance of the photodiode. Such a problem appears for the incident light with a high frequency region (e.g., a frequency of 10 GHz or 40 GHz) when the magnitude of incident light is large and the reverse bias voltage applied across the pn junction is low. That is the reason why the photodiode has the restricted frequency range.
It is an object of the present invention to provide a back-illuminated type light-receiving device with an improved frequency characteristic, which is usable for a higher frequency region.
The light-receiving device in accordance with the present invention comprises a semiconductor substrate and a semiconductor layer disposed on the front side of the substrate. The semiconductor layer includes a p-type layer and an n-type layer. The p-type layer contains a p-type dopant. The n-type layer is disposed above the p-type layer. An i-type layer may be disposed on the upper surface of the p-type layer, and the n-type layer may be disposed on the upper surface of the i-type layer. The p-type layer and n-type layer may be in contact with each other to form a pn junction therebetween. A light-receiving portion for receiving light entering the light-receiving device is provided on the back side of the substrate. The p-type layer and light-receiving portion are disposed on the opposite surfaces of the substrate, respectively. An anode may be in contact with the p-type layer. A cathode maybe in contact with the n-type layer.
For many of the electron-hole pairs in the light-receiving device in accordance with the invention, the distance that the electrons travel to reach the n-type layer is greater than the distance that the holes travel to reach the p-type layer. The drift velocity of an electron is faster than that of a hole. Therefore, the time difference for the concurrently generated electron and hole to reach the respective n-type and p-type layers can be reduced. Thus, the light-receiving device can respond to a wide range of the frequencies of incident optical signals.
In the light-receiving device, the light-receiving portion may be integrally formed on the substrate. In this case, the light-receiving device is easy to align with a light-emitting unit.
The light-receiving device may further comprise a diffusion suppressing layer of dopant between the substrate and p-type layer. The diffusion suppressing layer restrains diffusion of the p-type dopant from the p-type layer into the substrate. This can reduce the amount of the dopant-diffused portion to be removed from the substrate and eliminate the removal of the portion in making the light-receiving device.
Preferably, the light-receiving device is isolated. In this case, the light-receiving device is less likely to be electrically influenced by semiconductor crystal surfaces. The device isolation may be carried out by etching.
Preferably, the portion of the substrate into which the p-type dopant diffuses from the p-type layer is removed. In this case, the front side of the substrate may have first and second regions with different heights. The first region is higher than the second region. The p-type layer and n-type layer are disposed on the first region. The concentration of the p-type dopant in the second region is lower than that in the first region. The second region may be formed by removing the dopant-diffused portion from the substrate. The dopant-diffused portion may be removed by etching. For example, the dopant-diffused portion may be removed during etching for device isolation.
The diffusion suppressing layer of dopant may be an Si-doped layer. The p-type layer and n-type layer may have a mesa structure. The p-type layer may be provided by an epitaxial growth technique. The light-receiving portion may be an optical convex lens.
The dopant in the p-type layer may be carbon. In this case, the dopant diffusion into the substrate can be restrained.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications in the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.